It is a common knowledge that magnetic and mechanical properties of ferromagnetic materials, for instance steel, depend on their chemical composition and structure obtained during their manufacture. Therefore, manufacturing a steel possessing desirable melchanical properties, such as hardness, ultimate strength, yield strength, and elongation, may be ensured by maintaining the chemical composition constant and the manufacturing process stable. Since one and the same departure (within tolerance limits) from a chemical composition and the manufacturing process employed cause changes both in mechanical and magnetic properties, there exists a correlation dependence between these properties, which allow determining mechanical properties by measuring magnetic properties. The best magnetic properties for the above purpose are a coercive force and a remanent induction (or magnetization).
In a known in the art method entitled as "Method for measuring remanent magnetization" (cf. Japan accepted patent specification No. 53-29109, Int. Cl. G 01n 27/68) the test piece (plate) is passed between two electromagnets disposed in opposing relationship at the both sides of said plate, one at the top side and the other one at the underside. The electromagnets provides formation of a magnetic track in the test material, which magnetic track extends in the direction of movement of said test material. Located some distance from said electromagnets are transducers mounted for movement along a path crossing said magnetic track. The signal from the transducer is used to determine the quantity of remanent magnetization in the test plate. The inspection results in this method are influenced by displacement of the test material due to vibration, which affects the accuracy of measurement.
Furthermore, the presence of moving parts makes carrying out this method more difficult and impairs the reliability thereof.
In another known in the art method entitled as "Method and apparatus for detecting the presence of magnetic anomalies in ferromagnetic and non-ferromagnetic materials" (cf. U.K. Pat. No. 1,498,218, Int. Cl. G 01n 27/86) the test material is passed through a ring magnet so that said test material is magnetized by the magnet field. The parameters of the test material are determined by the magnitude of signals formed in the sensing coils under the action of the magnetic dipoles set up in the test material. The amplitudes of the thus produced signals representative of the properties of the test material are proportional to the magnitude of the magnetic dipoles and velocity of the test material.
The above-mentioned dependence of the output signal on the velocity of the test material affect the accuracy of the method.
There is also known a method for magnetic inspection of moving ferromagnetic articles of round section (cf. USSR Author's Certificate No. 696,369, 5.11.79) which comprises a pulse generator adapted to generate magnetizing pulses with a variable frequency of pulse repetition. The magnetizing means are made in the form of solenoids placed in antiparallel relationship in the circuit of the pulse generator. Inside one of the solenoids are mounted reading devices symmetric about the axis of said solenoid. As reading devices use is made of a convential ferroprobe-gradiometers which are placed in a circuit so that their output signals are summed. Field coils of the ferroprobes are connected through power amplifiers to an exciting circuit, and measuring coils are connected to the input of a selective amplifier tuned to the second harmonic.
Magnetic testing of the material for mechanical properties with the use of the above method is carried out as follows.
The test piece is passed inside the magnetizing solenoids and between the ferroprobe-gradiometers. A pulsed current is passed through the coils of the magnetizing solenoids to form magnetic marks. The parameters of said pulses are selected so as to form a maximum gradient of the remanent field. The frequency of repetition of the magnetizing pulses is selected depending on the velocity of the moving test piece so that while passing by the magnetic marks there is no remanent magnetic field inside the second solenoid. The ferroprobe-gradiometers are responsive to a gradient which is normal to the direction in which the remanent magnetic field component is moving, said gradient being representative of the mechanical properties of the test body. When the test piece in its motion between the ferroprobes is maintained evenly spaced therefrom their signals are equal, and the combined signal is equal to the sum of all the signals. In case the test piece is caused to displace from the center line of its movement path the strength of signals from the ferroprobes located at one side increases, while the strength of signals from the ferroprobes located at the other side decrease, but the sum of these signals (at low amplitudes of vibration) will be constant if the mechanical properties of the test body do not change.
The above method and device do not eliminate in a sufficiently wide range and with a required accuracy the influence of vibration on the accuracy of the test results, since the dependence of the residual field gradient on the distance to the surface of the test piece is non-linear.
Besides, the applicability of said method and device is limited to test pieces which are round in section.